Updated interpretation of magnetic anomalies and seafloor spreading stages in the south China Sea: Implications for the Tertiary tectonics of Southeast Asia

Abstract

We present the interpretation of a new set of closely spaced marine magnetic profiles that complements previous data in the northeastern and southwestern parts of the South China Sea (Nan Hai). This interpretation shows that seafloor spreading was asymmetric and confirms that it included at least one ridge jump. Discontinuities in the seafloor fabric, characterized by large differences in basement depth and roughness, appear to be related to variations in spreading rate. Between anomalies 11 and 7 (32 to 27 Ma), spreading at an intermediate, average full rate of •50 mm/yr created relatively smooth basement, now thickly blanketed by sediments. The ridge then jumped to the south and created rough basement, now much shallower and covered with thinner sediments than in the north. This episode lasted from anomaly 6b to anomaly 5c (27 to •16 Ma) and the average spreading rate was slower, •35 mm/yr. After 27 Ma, spreading appears to have developed first in the eastern part of the basin and to have propagated towards the southwest in two major steps, at the time of anomalies 6b-7, and at the time of anomaly 6. Each step correlates with a variation of the ridge orientation, from nearly E-W to NE-SW, and with a variation in the spreading rate. Spreading appears to have stopped synchronously along the ridge, at about 15.

Active thrusting and folding along the northern Tien Shan and Late Cenozoic rotation of the Tarim relative to Dzungaria and Kazakhstan

“…That such a great range rose so fast so recently supports the view that discrete tectonic phases and changes in strain styles have occurred as deformation migrated northwards across the Asian continent with the penetration of India into it Tapponnier et al , 1982]. More specifically, we fmd it tempting to relate the onset of significant shortening in the Tien Shan in the middle Miocene with the cessation of seafloor spreading in the South China sea (15-16 Ma [Taylor and Hayes, 1983;Briais et al, 1992]), the end of large-scale left-lateral shear along the Red River Ailao Shan zone (after =21 Ma Schllrer et al, 1990, Leloup, 1991), the cooling of highgrade Tertiary gneisses along that zone (from ""19 Ma onwards, (M. Harrison et al, unpublished manuscript, 1991)), as well as with the lower Miocene peak metamorphism on the MCT ( =20 Ma [e.g., Hubbard and Harrison, 1989]), the fast cooling pulse in the Gangdese Shan of southern Tibet ( =20 Ma [Copeland et al, 1987]), and the onset of massive turbidite flux in the distal Bengal fan (somewhat prior to ""17 Ma [Cochran, 1987]), all of which may imply increased shortening, uplift and erosion rates in the Himalayas at that time, possibly correlative of the most rapid change in seawater Sr isotopic ratio, between 20 and 15 Ma [Richter et al, 1992]. That there is growing evidence for a rough coevality between all these different events implies that they may all result from a single cause.…”

Tracing the life history of a marginal sea—On “The South China Sea Deep” Research Program

“…The age estimations were given at the best scientific level at that time, but undoubtedly suffer from uncertainty in interpretation due to the lack of diurnal variation correction, to limitations in instrumental precision and positioning accuracy, and to the remarkable noise disturbance. Later, the original age model was revised and improved by French [3,4], Chinese [5,6] and German [7] authors, resulting in new versions of interpretation but meanwhile also in increased uncertainty (Table 1; Figure 1). …”

“…The relationship between the two sub-basins inspires the first divergence of opinions on the seafloor spreading of the SCS. It is widely believed that the Eastern Sub-Basin formed first before propagating towards southwest with a jump of spreading ridge [4]. But an alternative view believes that the Southwestern Sub-Basin may have predated the Eastern one [5,8] (Table 1).…”

“…According to Briais et al (1993) [4], the spreading ridge of the SCS jumped southwards and led to the initiation of seafloor spreading in the SW Sub-Basin during 7/6b about 25-23 Ma. On the basis of the abrupt changes in Nd isotopic and numerous other geochemical indices in sediments above this level, Li et al (2002) [13] postulated that the sediment provenance changed from Indonesia and Borneo in the SW to the China continent in the North at ca.…”

Earthquake Swarm Recorded by an Ocean Bottom Seismic Array in Southwest Offshore of Taiwan in October, 2005

“…With the identification of magnetic lineation in the SCS basin, the extension of the SCS basin occurred during 32 to 15 Ma before present (magnetic anomaly C11 -C5c) (Taylor and Hayes 1980;Briais et al 1993), and the continent-ocean boundary (COB) reached the latitude of 19°N in SW off Taiwan. Recent studies with new magnetic and gravity surveys indicate that the COB for the SCS and the SE Asia could extend further to the north, reaching 20°N Wu et al 2004) and the oldest age of SCS opening is about 37.8 Ma .…”